pc2 have been running Buster and 10.4 for a few weeks now and the spare hosts (pc1010 and pc2010) have just been upgraded.
Let's upgrade pc1 and pc3 also so we can have the full pc infra running Buster and 10.4
This involves MW depooling.
My suggestion would be to upgrade:
pc1: first codfw, then eqiad
pc3: first codfw, then eqiad